1. Field of the Invention
The present invention relates to a solid-state radiation detector of the type having a pixel matrix that supplies an output signal dependent on the incident radiation, having a conversion layer for converting the incident radiation into charge, a storage capacitor for storing the charge and a transistor for reading out the charge, as well as to a medical examination and/or treatment device with such a solid-state radiation detector,
2. Description of the Prior Art
Use is increasingly being made in X-ray technology of digital solid-state radiation detectors, also termed matrix detectors, that are distinguished chiefly by a linear response. This means that the output signal supplied by the detector or by the respective pixel, i.e. an image point, is proportional over very wide ranges to the incident X-ray and radiation dose. However, the design imposes a maximum radiation dose, which if exceeded causes the detector to suddenly reach its limitations. The reason for this can reside in the analog signal processing or with the downstream analog-to-digital converter that converts the analog output signals of the pixels into digital signals. This response is unfavorable, since the aforementioned abrupt transition can lead to artifacts in the X-ray images that are of course not desired.
Known solid-state radiation detectors for this reason usually are driven only at a low levels, for example at 5% to 10% of the maximum drive level, in order to avoid local overdriving due to brief high incident radiation doses as in the case of unattenuated radiation. However, for relatively low radiation doses, this disadvantageously results in a worsening of the signal-to-noise ratio, particularly when the quantization (step division) by the analog-to-digital converter cannot be neglected.
A solid-state radiation detector of this known (disadvantageous) type is disclosed in U.S. Pat. No. 5,598,004. For the same purpose of eliminating dynamic range limiting in the case of a two-dimensional radiation detector, JP Abstract 2000 11 16 52 A describes the connection of an external resistance between a biasing voltage supply and a biasing electrode of a semiconductor layer. Across this resistance a voltage drop occurs in the voltage at the biasing electrode, the result being a decrease in the electric field generated in the semiconductor, causing fewer charge carriers to be produced in the semiconductor. Reference may also be made to the publication by G. F. G. Delaney and E. C. Finch entitled xe2x80x9cRadiation Detectorsxe2x80x9d, Clarendon Press, Oxford 1992, pages 299xe2x96xa10311 as radiation detectors relating to the general prior art.
It is an object of the present invention to provide a solid-state radiation detector and a medical examination and/or treatment device with such a solid-state radiation detector wherein the aforementioned disadvantages of known detectors are avoided.
This object is achieved in accordance with the invention in a solid-state radiation detector of the type initially described wherein the capacitance of the storage capacitor is so small that because of the voltage drop across the storage capacitor, the output signal of the pixel exhibits, starting from a specific value of the incident radiation dose, a sublinear response with respect to the radiation dose.
At the conversion layer of a solid-state radiation detector of the type described above, a voltage is present having a magnitude that depends on the thickness of the conversion layer, usually a selenium layer, and which serves to separate the charges produced by the incident radiation, in relation to the active pixel matrix with the storage capacitor. This voltage is in the kV range, which lies above the field strengths at the conversion layer, for example in the range of 10 V/xcexcm. Thus, with a layer thickness of, for example, 200 xcexcm an applied voltage of approximately 2 kV is assumed for the purpose of achieving this field strength. The capacitance of the storage capacitor is selected in this case to be sufficiently large for known solid-state radiation detectors so that the voltage at the conversion layer always remains approximately equal to the externally-applied voltage.
The invention departs from this known design of the capacitance of the storage capacitor and, by contrast, reduces the capacitance of the storage capacitor Ci,j so much that the change in voltage dU=qi,j/Ci,j, which is caused by the signal charge qi,j at the pixel (i,j), is in the order of magnitude of the applied voltage. Thus a substantial fraction of the voltage drops across the storage capacitor, and the effective voltage Veff drop at the conversion layer, in particular in the case of high irradiated X-ray doses due to unattenuated radiation, for example, is reduced in accordance with the relationship Veff=Vxe2x88x92qi,j/Ci,j, wherein V is the externally-applied voltage. Thus, specific use is made of the voltage divider properties of the combination of the conversion layer and the storage capacitor in order to produce an intended reduction in the effective voltage Veff across the conversion layer.
The ability to produce electron/hole pairs in the conversion layer, however, depends on the strength of the electric field across the conversion layer, the result generally being a linear relationship as described, for example, in the publication entitled xe2x80x9cNew Digital Detector for Projection Radiographyxe2x80x9d by Lee, Cheung, Jeromin, in Physics of Medical Imaging, Proc. SPIE No. 2432, pages 237 ff and in FIG. 8 there.
If the voltage drop across the conversion layer is now lowered because of the voltage drop across the storage capacitor, the effective field strength across the conversion layer also is reduced, necessarily resulting in fewer electron/hole pairs being generated and collected. Consequently, with an increasing X-ray dose the sensitivity of the solid-state radiation detector is reduced within an appropriate time, starting from a specific dose, before the end of the drive-level range so as to advantageously avoid a sudden limitation of the output signal. Because of the lower number of electrons collected, the originally linear response of output signal in relation to radiation dose is cancelled, and a sublinear response is produced in such a way that the output signal responds in a less than proportional fashion to the radiation dose.
The solid-state radiation detector according to the invention therefore permits a substantially higher drive level with particular advantage, since no overdriving will occur, due to the sublinear response and the lower sensitivity achieved even in the case of high incident doses, for example in the case of unattenuated radiation. The result is that a higher gain can be selected so that the output signals can be increased in the range of low doses, and a substantially improved signal-to-noise ratio thus is achieved.
In an embodiment of the invention the capacitance of the storage capacitor is selected so that the ratio of the maximum voltage drop across the storage capacitor to the maximum voltage dropping across the conversion layer is at least 1:10 or greater, in particular, at least 1:5 or greater. In any case, the voltage drop across the storage capacitor should be of essentially the order of magnitude of the voltage applied from outside. With a voltage of 2 kV, for example, at the conversion layer, the voltage dropping across the storage capacitor should be, for example, at least 500 V or even more.
If it is expected that the voltage drop across the storage capacitor will be very high, it is expedient in order to avoiding damage to the readout transistors, to design these readout transistors as high-voltage transistors so that they can switch voltages of several 100 V without their parameters being changed or damaged.
The conversion layer itself is expediently a selenium layer, the transistor is a thin-film transistor (TFT) based on amorphous silicon (a-Si), or is a high-voltage thinfilm transistor (HVTFT) based on a-Si. Such high-voltage thin-film transistors are known, for example, from the publication by Tuan in Materials Research Society Symposium Proceedings Vol. 70, page 651 ff from 1986.
In addition to the solid-state detector itself, the invention also relates to a medical examination and/or treatment device having at least one radiation source and at least one solid-state radiation detector of the type described above, as well as a processing device to which the output signals from the solid-state radiation detector are supplied, the processing device being designed to compensate for the sublinearity of the output signals, in order to provide a detector with a relatively large dynamic range. The compensation can be performed, for example, analytically by using appropriate analytical compensation means, or by using at least one look-up table in which appropriate compensation values are entered.